Electro-optic media and writable display incorporating the same

ABSTRACT

An electro-optic device comprising electrophoretic medium including a dispersion of a plurality of particles in a fluid configured to migrate within the fluid in a direction responsive to an applied electric field. The plurality of particles include a first type of particles having a first charge of a first charge polarity, a second type of particles having a second charge of a second charge polarity, and a third type of particles having a third charge of the second charge polarity. The first charge polarity is opposite to the second charge polarity, and the third type of particles are configured to migrate within the fluid in a direction responsive to an applied magnetic field gradient.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/752,614, filed Oct. 30, 2018, which is incorporated herein byreference in its entirety.

BACKGROUND

The technology described herein relates to an electro-optic devicecomprising electrophoretic medium containing charged particles andmagnetically responsive particles that can be addressed with aspecialized instrument (stylus or print head) and related methods.

The entire contents of all U.S. patents and published Applicationsmentioned below are herein incorporated by reference.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme, which only drives pixels to their two extremeoptical states with no intervening gray states.

Some electro-optic materials are solid in the sense that the materialshave solid external surfaces, although the materials may, and often do,have internal liquid- or gas-filled spaces. Such displays using solidelectro-optic materials may hereinafter for convenience be referred toas “solid electro-optic displays”. Thus, the term “solid electro-opticdisplays” includes rotating bichromal member displays, encapsulatedelectrophoretic displays, microcell electrophoretic displays andencapsulated liquid crystal displays.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequate servicelife for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC. and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. Typically, thecapsules are themselves held within a polymeric binder to form acoherent layer positioned between two electrodes. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained with in aplurality of cavities formed within a carrier medium, typically apolymeric film. The technologies described in these patents andapplications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 7,002,728, 7,679,814, 9,759,980, and 6,870,661;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. Nos. 6,922,276 and 7,411,719;

(c) Microcell structures, wall materials, and methods of formingmicrocells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;

(d) Methods for filling and sealing microcells; see for example U.S.Pat. Nos. 7,144,942 and 7,715,088;

(e) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178 and 7,839,564;

(f) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318 and7,535,624;

(g) Color formation and color adjustment; see for example U.S. Pat. Nos.7,075,502 and 7,839,564;

(h) Methods for driving displays; see for example U.S. Pat. Nos.7,012,600 and 7,453,445;

(i) Applications of displays; see for example U.S. Pat. Nos. 7,312,784and 8,009,348 and U.S. Patent Application Publication No.US2017/0336896; and

(j) Non-electrophoretic displays, as described in U.S. Pat. No.6,241,921 and U.S. Patent Application Publication No. 2015/0277160; andapplications of encapsulation and microcell technology other thandisplays; see for example U.S. Patent Application Publications Nos.2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A variety of systems is available for addressing an electro-opticdisplay with a stylus. For example, a stylus can be used to take noteson certain tablets. As a user passes the stylus over a surface of theelectro-optic display, the electro-optic display activates pixelscorresponding to those over which the stylus passed, based on theposition of the stylus detected by the electro-optic display. Somemagnetically responsive displays may be addressed with a stylus thatcontains a magnet and/or produces a magnetic field.

Current writable electro-optic display systems are limited in the numberof colors that may be provided using an electrophoretic medium.Accordingly, there is a need for improved electrophoretic-based writabledevices that enable a user to produce images having multiple colors.

SUMMARY

In a first aspect of the present invention, an electro-optic devicecomprises a first surface on a viewing side, a second surface on theopposite side of the first surface, an electrophoretic medium disposedbetween a light-transmissive electrically conductive layer and an arrayof pixel electrodes and comprising a plurality of particles in a fluid,the plurality of particles comprising (1) a first type of particleshaving a first color, a first charge of a first charge polarity, (2) asecond type of particles having a second color, a second charge of asecond charge polarity, and (3) a third type of particles having a thirdcolor, a third charge of the second charge polarity, wherein the first,second and third colors are different from one another, the first chargepolarity is opposite to the second charge polarity, the plurality ofparticles are configured to migrate within the fluid in a directionresponsive to an applied electric field, the third type of particles areconfigured to migrate within the fluid in a direction responsive to anapplied magnetic field gradient, the second type of particles have anelectric field threshold, such that (a) application of a voltagepotential difference between the light-transmissive electricallyconductive layer and a pixel electrode to generate an electric fieldstronger than the electric field threshold and having a polarity drivingthe second type of particles adjacent to the light-transmissiveelectrically conductive layer, will cause a pixel corresponding to thepixel electrode to display the second color at the first surface, (b)application of a voltage potential difference between thelight-transmissive electrically conductive layer and a pixel electrodeto generate an electric field stronger than the electric field thresholdand having a polarity driving the first type of particles adjacent tothe light-transmissive electrically conductive layer, will cause a pixelcorresponding to the pixel electrode to display the first color at thefirst surface, (c) once the first color is displayed at the firstsurface, application of a voltage potential difference between thelight-transmissive electrically conductive layer and a pixel electrodeto generate an electric field weaker than the electric field thresholdhaving a polarity driving the third type of particles adjacent to thelight-transmissive electrically conductive layer, will cause a pixelcorresponding to the pixel electrode to display the third color,

In another aspect of the present invention, a method of operating anelectro-optic device, wherein the electro-optic device comprises (a) afirst surface on a viewing side, (b) a second surface on the oppositeside of the first surface, (c) an electrophoretic medium disposedbetween a light-transmissive electrically conductive layer and an arrayof pixel electrodes and comprising a plurality of particles in a fluid,the plurality of particles comprising (1) a first type of particleshaving a first color, a first charge of a first charge polarity, (2) asecond type of particles having a second color, a second charge of asecond charge polarity, and (3) a third type of particles having a thirdcolor, a third charge of the second charge polarity, wherein the first,second and third colors are different from one another, wherein thefirst charge polarity is opposite to the second charge polarity, whereinthe plurality of particles are configured to migrate within the fluid ina direction responsive to an applied electric field, and wherein thethird type of particles are configured to migrate within the fluid in adirection responsive to an applied magnetic field gradient, comprisesthe steps of: (A) contacting a first location on a first surface of thedevice with a stylus comprising a magnetic tip to cause a third type ofparticles to migrate towards the first location, and (B) contactingagain the first location on the first surface of the device with thestylus comprising a magnetic tip to cause a second type of particles tomigrate towards the first location.

The aspects and embodiments described above, as well as additionalaspects and embodiments, are described further below. Other aspects ofthe present invention will be apparent in view of the followingdescription. These aspects and/or embodiments may be used individually,all together, or in any combination of two or more, as the applicationis not limited in this respect.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1 illustrates a cross-sectional side view of an addressableelectro-optic display in combination with a first stylus according to afirst embodiment of the present invention.

FIG. 2 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 1 in combination with a second stylus.

FIG. 3 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 1 in combination with a third stylus.

FIG. 4 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 1 displaying a first optical state.

FIG. 5 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 1 displaying a second optical state.

FIG. 6 illustrates a cross-sectional side view of the addressableelectro-optic display in the white state.

FIG. 7 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 6 in combination with a second stylus(contacting the stylus once).

FIG. 8 illustrates a cross-sectional side view of the addressableelectro-optic display of FIG. 6 in combination with a second stylus(contacting the stylus more than once).

FIG. 9 shows a photograph of an addressable electro-optic displaycontacted by a magnetic stylus to write a phrase and to make a drawing.

DETAILED DESCRIPTION

Various embodiments of the present invention relate to electrophoreticmedia and particle-based electro-optic displays, which are electricallyand magnetically addressable. The various embodiments of the presentinvention may be configured to provide both global and local addressingcapabilities. The global addressing capability may be used to create asolid color state, for example white or black, and therefore beconsidered an “erase” state. The global addressing state may beelectrically controllable. For example, the display may includeelectrodes on opposing sides of a particle-based electro-optic layer ofthe display, and the electrodes may be operated to create a suitableelectric field to set the ink to a uniform color state. The display mayinclude a controller for controlling the electric field presented to theelectrophoretic particles. The controller may apply a static electricfield or a time-dependent electric field, i.e., a waveform. The localaddressing capability may be provided by one or more writing implementscreating an electric or magnetic field. The term “writing implement” asused herein includes a stylus.

The terms used herein that refer to particles as “magnetic” and“magnetically responsive” and “responsive to a magnetic field gradient”and “magnetically addressable” and “configured to migrate within thefluid in a direction responsive to an applied magnetic field gradient”are synonymous. They refer to particles that migrate in the liquiddispersion medium upon the application of a magnetic field gradient andmay form chains. Such particles may be referred in the scientificliterature as magnetophoretic particles.

According to one embodiment of the present invention, an electrophoreticmedia may comprise a dispersion of a plurality of charged particles. Theplurality of electrophoretic particles preferably include at least threedifferent sets of particles: a light-scattering white pigment, alight-scattering, magnetic, colored pigment, and a black pigment. Thecolored pigment may be any color other than white or black. In apreferred embodiment, the colored pigment is red or yellow. In order tocontrol the mobility of each set of charged particles through thedispersion fluid, one or more of the sets of particles may include acore pigment having a polymer coating, typically a polymer grafted oradsorbed to the surface of the pigment particles, such as thosedescribed in U.S. Patent Application Publications 2015/0103394 and2016/0085121. The diameter of the core pigments and/or thickness of thecoatings may differ between the sets of charged particles.

In a preferred embodiment of the present invention, the black pigmentand the light-scattering, magnetic, colored pigment have the same chargepolarity, while the white pigment has the opposite polarity, and onlythe light-scattering, magnetic, colored pigment is responsive to amagnetic field gradient, i.e. moves in a direction within the dispersionfluid in response to an applied magnetic field gradient.

The core pigment used in the white particle may be a metal oxide of highrefractive index as is well known in the art of electrophoreticdisplays. Examples of materials for the white particle include, but arenot limited to, inorganic pigments, such as TiO₂, ZrO₂, ZnO, Al₂O₃,Sb₂O₃, BaSO₄, PbSO₄ or the like.

The materials used as the core pigment for the black particle include,but are not limited to, CI pigment black 26 or 28 or the like (e.g.,manganese ferrite black spinel or copper chromite black spinet) orcarbon black.

The colored pigment particles are preferably non-black and non-white andmay be of a color such as red, green, blue, magenta, cyan or yellow. Thematerials used as the core pigment for the light-scattering, magnetic,colored pigment include, but are not limited to, CI pigment PR 254,PR122, PR1.49, PG36, PG58, PG7, PB28, PB15:3, PY138, PY150, PY155 orPY20. Those are commonly used organic pigments described in color indexhandbook “New Pigment Application Technology” (CMC Publishing Co, Ltd,1986) and “Printing Ink Technology” (CMC Publishing Co, Ltd, 1984).Specific examples include Clariant Hostaperm Red D3G 70-EDS, HostapermPink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm BlueB2G-EDS, Hostaperm Yellow H4G-EDS, Hostaperm Green GNX, BASF Irgazinered L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD; SunChemical phthalocyanine blue, phthalocyanine green, diarylide yellow ordiarylide AAOT yellow. Examples of composite magnetic particles mayinclude the materials described in U.S. Pat. No. 7,130,106.

In addition to the colors, the first, second and third types of chargedparticles may have other distinct optical characteristics, such asoptical transmission, reflectance, luminescence or, in the case ofdisplays intended for machine reading, pseudo-color in the sense of achange in reflectance of electromagnetic wavelengths outside the visiblerange.

The plurality of charged pigment particles may carry a natural charge orare charged through the presence of a charge controlling agent.

The percentages of the charged particles in the fluid may vary. Forexample, in a dispersion containing three types of particles, the blackparticle may take up about 0.1% to 10%, preferably 0.5% to 5% by volumeof the electrophoretic fluid; the white particle may take up about 1% to50%, preferably 5% to 15% by volume of the fluid; and the coloredparticle may take up about 2% to 20%, preferably 1% to 10% by volume ofthe fluid.

The electrophoretic media made according to the various embodiments ofthe present invention may further include one or more other optionalcomponents, such as uncharged neutral buoyancy particles, such as thosedescribed in US Patent Application 2015/0103394, charge control agents,and surfactants.

The dispersion fluid in which the plurality of particles are dispersedmay be a clear and colorless solvent. The solvent preferably has a lowviscosity and a dielectric constant in the range of about 2 to about 30,preferably about 2 to about 15 for high particle mobility. Examples ofsuitable dielectric solvent include hydrocarbons such as Isopar,decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, silicon fluids, aromatic hydrocarbons such as toluene,xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene,halogenated solvents such as perfluorodecalin, perfluorotoluene,perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane orpentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 orFC-5060 from 3M Company, St. Paul Minn., low molecular weight halogencontaining polymers such as poly(perfluoropropylene oxide) from TCIAmerica, Portland, Oreg., poly(chlorotrifluoro-ethylene) such asHalocarbon Oils from Halocarbon Product Corp., River Edge, N.J.,perfluoropolyalkylether such as Gulden from Ausirnont or Krytox Oils andGreases K-Fluid Series from DuPont, Del., polydimethylsiloxane basedsilicone oil from Dow-coming (DC-200).

As noted above, two types of charged pigment particles in a preferredembodiment of the present invention may carry opposite chargepolarities. Furthermore, the third type of charged pigment particles maybe slightly charged. The term “slightly charged” is intended to refer tothe charge level of the particles less than about 50%, preferably about5% to about 30%, of the charge intensity of the stronger chargedparticles. In one embodiment, the charge intensity may be measured interms of zeta potential. In one embodiment, the zeta potential isdetermined by Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signalprocessing unit, ESA EN# Attn flow through cell (K:127). The instrumentconstants, such as density of the solvent used in the sample, dielectricconstant of the solvent, speed of sound in the solvent, viscosity of thesolvent, all of which at the testing temperature (25° C.) are enteredbefore testing. Pigment samples are dispersed in the solvent (which isusually a hydrocarbon fluid having less than 12 carbon atoms), anddiluted to between 5-10% by weight. The sample also contains a chargecontrol agent (Solsperse® 17000, available from Lubrizol Corporation, aBerkshire Hathaway company), with a weight ratio of 1:10 of the chargecontrol agent to the particles. The mass of the diluted sample isdetermined and the sample is then loaded into the flow through cell fordetermination of the zeta potential.

The term “threshold voltage” or “electric field threshold”, in thecontext of the present invention, is defined as the maximum electricfield that may be applied for a period of time (typically not longerthan 30 seconds, preferably not longer than 15 seconds), to a group ofparticles, without causing the particles to appear at the viewing sideof a pixel, when the pixel is driven from a color state different fromthe color state of the group of particles. The term “viewing side”, inthe present application, refers to the first surface in a display layerwhere images are seen by the viewers.

The threshold voltage or electric field threshold is either an inherentcharacteristic of the charged particles or an additive-induced property.

In the former case, the threshold voltage or electric field threshold isgenerated, relying on certain attraction force between oppositelycharged particles or between particles and certain substrate surfaces.

In the case of additive-induced threshold voltage or electric fieldthreshold, a threshold agent, which induces or enhances the thresholdcharacteristics of an electrophoretic fluid, may be added. The thresholdagent may be any material, which is soluble or dispersible in thesolvent or solvent mixture of the electrophoretic fluid and carries orinduces a charge opposite to that of the charged particles. Thethreshold agent may be sensitive or insensitive to the change of appliedvoltage. The term “threshold agent” may broadly include dyes orpigments, electrolytes or polyelectrolytes, polymers, oligomers,surfactants, charge controlling agents and the like. Additionalinformation relating to the threshold agent may be found in U.S. Pat.No. 8,115,729.

In one example, the electrophoretic medium may contain white pigmentparticles having a first charge of negative polarity, black pigmentparticles having a second charge of positive polarity and red pigmentparticles having a third charge of positive polarity. If the red pigmentparticles is slightly charged, as defined above, that is, the redpigment particles have weaker charge compared to the charges on theblack and the white pigment particles, the black particles may require a“threshold voltage” to be applied across the electrophoretic medium inorder to move to the viewing side of the electrophoretic medium, asopposed to the lower voltage required for the red particles to move toviewing side of the electrophoretic medium. The strong electrostaticattraction between the black particles and the white particles, which issmaller than the electrostatic attraction between the red particles andthe white particles, causes the migration of the red particles adjacentto the light-transmissive electrically conductive layer when the appliedvoltage potential difference between the light-transmissive electricallyconductive layer and a pixel electrode to generate an electric fieldweaker than the electric field threshold and having a polarity drivingthe red particles adjacent to the light-transmissive electricallyconductive layer. Application of voltage potential difference betweenthe light-transmissive electrically conductive layer and a pixelelectrode to generate an electric field stronger than the electric fieldthreshold (and having a polarity driving the black particles adjacent tothe light-transmissive electrically conductive) is sufficient toovercome the electrostatic attraction between the white and blackparticles and causes the migration of the black particles adjacent tothe light-transmissive electrically conductive layer.

The three types of charged particles may have varying sizes. In oneembodiment, one of the three types of particles is larger than the othertwo types. For example, both the black and the white particles may berelatively small and their sizes (tested through dynamic lightscattering) may range from about 50 nm to about 800 nm and morepreferably from about 200 nm to about 700 nm, and the colored particlespreferably are about 2 to about 50 times and more preferably about 2 toabout 10 times larger than the black particles and the white particles.The values correspond to the average diameters of the correspondingpigment particles.

As explained above, the electrophoretic media according to the variousembodiments of the present invention may be incorporated into a writableelectro-optic display that utilizes one or more writing implements. Thewriting implements may be hand-held. At least one of the writingimplements may produce an electric and/or a magnetic field that causes achange in an optical state of the display within an area local to thewriting implement. The change in optical state may include movement ofthe white, colored, and/or black particles within the display.

Referring now to FIG. 1, a display according to one embodiment of thepresent invention may comprise a dispersion comprising a plurality ofelectrophoretic particles located between a first conductive layer 100and a second conductive layer 102. As illustrated in FIG. 1, the displayis viewed from above; therefore, the first conductive layer 100 may be acontinuous layer of light-transmissive conductive material, such asindium tin oxide. The second conductive layer 102 may or may not belight transmissive. The layer 102 may be provided, for example, in theform of a substrate comprising an array of pixel electrodes, such as aTFT array. The display may further comprise touch-sensitive layer 101,such as a touch sensor, for sensing contact by a writing implement 114.

The electrophoretic particles may comprise, for example, negativelycharged white pigment particles 104, positively charged black pigmentparticles 108, and positively charged, light-scattering, magnetic,colored pigment particles 106. As would be appreciated by one of skillin the art, the charge polarities of the particles may, in some example,be reversed, such that the white pigments is positively charged, whilethe black and magnetic particles are negatively charged. If the writabledisplay is intended to include a highlighting function, the color of themagnetic particles 106 may be red, for example.

Referring now to FIGS. 1 to 3, the display according to one embodimentof the present invention may be combined with three different styli,110, 112 and 114. Stylus 110 may be a magnetic stylus, used for writingthe highlight color. Stylus 112 may be non-magnetic and used for writingblack, while stylus 114 may be non-magnetic and used for writing white.Alternatively, all three of these functions may be incorporated into asingle writing implement. For example, a single stylus may have onemagnetic end for highlighting and an opposite end capable of switchingbetween writing in either black or white. In FIGS. 1 to 3 the styli areillustrated with similarly shaped hemispherical tips; however, a writingtip of a writing implement as described herein is not limited to anyparticular shape. For example, each stylus may have a characteristic tipshape that may be detected by the touch-sensitive layer, so that thedisplay recognizes which stylus is being used.

For all three stylus options, the touch-sensitive layer 101 may not onlysense, but also record the location of writing, so that this informationmay be digitized and stored in memory, such that the written image maybe saved, retrieved, and displayed at the user's discretion.

According to one method of operating a display according to the presentinvention, the optical states displayed at various locations on thedisplay may be controlled with or without a writing implement. A firstoptical state illustrated in FIG. 1 may be achieved by contacting asurface of the display with stylus 114, such that the touch-sensitivelayer 101 recognizes stylus 114 and its location. At this location, avoltage is applied between electrode layers 100 and 102, such thatelectrode layer 102 is negative relative to electrode layer 100. As aresult, the white particles 104 are driven towards the viewing side ofthe display to display a white optical state about the area of contactby the stylus 114. For example, the negative voltage may be appliedbetween electrode layer 100 and only the pixel electrodes within an areaof electrode layer 102 corresponding to the position of stylus 114. Inthis manner, stylus 114 may be used as an “erasing stylus” to achievelocal erasure of a black or a red image, for example. A global erase maybe achieved, for example, without the stylus by simultaneously applyinga negative voltage between every pixel electrode within electrode layer102 and electrode layer 100.

A second optical state illustrated in FIG. 2 may be achieved bycontacting a surface of the display with stylus 110. As previouslyexplained, the type and location of stylus may be detected by thetouch-sensitive layer 101. When stylus 110 is recognized, theelectrophoretic fluid is not switched electrically (i.e., no voltagedifference is applied between electrodes 100 and 102). Rather, thecolored magnetic particles 106 move in the magnetic field gradientgenerated by the proximity of the stylus 110. Such, motion typicallyprovides “chained-particle” states in which some of the particleslocated at the viewing surface (e.g. white pigment particles 104 in FIG.2) have been displaced by the colored particles 106.

In some embodiments, the magnet incorporated in stylus 110 may be apermanent magnet. The magnet may be of any suitable type, including butnot limited to neodymium iron boron, samarium cobalt, alnico, ceramicand ferrite magnets, or combinations thereof. While the magnet may belocated in any portion of the stylus, the magnet is preferably orientedsuch that its magnetic field is co-aligned with the tip of the stylus.According to some embodiments, the magnet may be an electromagnet. Insuch cases, a suitable power source may be located within orelectrically connected to the stylus and the magnet within the stylus.Furthermore, the magnet may have any suitable shape, including a cuboidor a ring shape. According to some embodiments, the magnet will producea field gradient strength of between approximately 10 and 50 Gauss onthe magnetic particles.

A third optical state illustrated in FIG. 3 may be achieved bycontacting a surface of the display with stylus 112, such that thetouch-sensitive layer 101 recognizes stylus 112 and its location. Atthis location, a voltage is applied between electrode layers 100 and102, such that electrode layer 102 is positive relative to electrodelayer 100. As a result, the black particles 104 are driven towards theviewing side of the display to display a black optical state about thearea of contact by the stylus 112. For example, the positive voltage maybe applied between electrode layer 100 and only the pixel electrodeswithin an area of electrode layer 102 corresponding to the position ofstylus 112. In this manner, stylus 112 may be used as a “writingstylus,” so that a user may use the display for note taking, forexample.

In a preferred embodiment, the black pigment particles 108 may have asmaller diameter than that of the light-scattering, magnetic, coloredparticles 106, which have the same charge polarity. For example,referring again to FIG. 3, if both the black pigment particles 108 andcolored magnetic particles 106 are positively charged, a positivevoltage may be applied of sufficient strength and/or duration to enablethe smaller black particles 108 to migrate to the viewing side of thedisplay through the spaces between the magnetic particles 106 andobscure the colored particles 106.

In order to retrieve a stored image that includes pixels having anoptical state provided by the colored magnetic particles, it may bepreferred that the colored magnetic particles have a higher mobilitythan the similarly charged black particles. This may be accomplished byapplying different polymer coatings to the colored and black particlesand/or providing slightly charged black particles. For example,referring to FIG. 4, a voltage is applied between electrode layers 100and 102, such that electrode layer 102 is positive relative to electrodelayer 100. However, the strength and/or duration of the applied voltageis below a pre-selected threshold, such that colored magnetic particles106 are driven to the viewing side of the display, but not the whiteparticles 104 or black particles 108, which may be aggregated together.To provide pixels within a stored image having an optical state providedby the white or black particles, a negative voltage of sufficientstrength and/or duration may be applied to drive the white particles 104to the viewing surface or a positive voltage of sufficient strength andor duration may be applied to drive the black particles 108 between thecolored particles 106 and to the viewing surface (as illustrated in FIG.5).

Table 1 below summarizes an example of the various modes ofwriting/retrieving an image on a display according to an embodiment ofthe present invention. W represents white, K represents black, and Rrepresents colored magnetic particles.

TABLE 1 Initial State Final State Modality Method W K Writing Touchstylus, TFT/high voltage (+) W R Writing Magnetic stylus, no electricaladdressing K or R W Local erase Touch stylus, TFT/high voltage (−) K orR W Global erase TFT/high voltage (−) W K Recall TFT/high voltage (+) WR Recall TFT/low voltage (+)

The electro-optic device, which comprises a first type of particleshaving a first color, a first charge of a first charge polarity(positively-charged white particles in the example corresponding to thetable), a second type of particles having a second color, a secondcharge of a second charge polarity (negatively-charged black particles,which have an electric field threshold, in the example corresponding tothe table), a third type of particles having a third color, a thirdcharge of the second charge polarity (negatively-charged, red particlesconfigured to migrate within the fluid in a direction responsive to anapplied magnetic field gradient, in the example corresponding to thetable), can be operated by a method comprising the steps of (A)contacting a first location on the first surface of the device with oneof a first and second stylus, (B) applying an electric field to causeone of the first type (white) and second type (black) of particles tomigrate towards the first location, (C) contacting a second location onthe first surface of the display with one of the first and secondstylus, and (D) applying a magnetic field gradient to cause the thirdtype of particles (red) to migrate towards the second location.

In another embodiment, the electro-optic device comprises anelectrophoretic medium comprising negatively-charged white particles604, positively-charged red particles 606, and positively-charged blackmagnetic particles 608. It was observed that, starting from the whitestate (FIG. 6), and contacting the writable surface of the electro-opticdisplay with a magnetic stylus 110 once, a grey image is created. Themagnetic stylus attracted and aligned the magnetic black particles 608,as graphically illustrated in FIG. 7. After the magnetic stylus wasapplied more than one times at the same location of the writable surfaceof the electro-optic device where the grey color existed, the color ofthe location changed to red. This is graphically illustrated in FIG. 8and in the photograph of FIG. 9. The electro-optic device 900 of FIG. 9,which was original in its white state 901, was contacted by a magneticstylus and the phrase “magnetic addressing” 902 was written on it. Thephrase appears grey. In another location of the electro-optic device900, a magnetic stylus was used to make a drawing 903. The styluscontacted the electro-optic device multiple times on the location of thedrawing 903. The drawing 903 appears to be in red color. Theelectrophoretic medium of the embodiment was prepared using Isopar E asthe electrophoretic fluid, which comprises (a) Solsperse 19000 (suppliedby Lubrizol) as a charge control agent, (b) positively-charged ironoxide black magnetic pigment (Pigment Black 11), (c) negatively-chargedtitanium dioxide white pigment (Pigment White 6), and (d)negatively-charged red (Pigment Red 254). In the case where the startingoptical state is black, contacting the writable surface of theelectro-optic display with a magnetic stylus once, a red image wascreated, which becomes a brighter red after application of magneticstylus more than one times at the same location of the writable surfaceof the electro-optic device. FIG. 9 shows the demonstrates the display

This embodiment describes an electro-optic device that comprises (a) anelectrophoretic medium disposed between a light-transmissiveelectrically conductive layer and an array of pixel electrodes andcomprising a plurality of particles in a fluid, the plurality ofparticles comprising (1) a first type of particles having a first color,a first charge of a first charge polarity, (2) a second type ofparticles having a second color, a second charge of a second chargepolarity, and (3) a third type of particles having a third color, athird charge of the second charge polarity, wherein the first, secondand third colors are different from one another, wherein the firstcharge polarity is opposite to the second charge polarity, wherein theplurality of particles are configured to migrate within the fluid in adirection responsive to an applied electric field, and wherein the thirdtype of particles are configured to migrate within the fluid in adirection responsive to an applied magnetic field gradient. Thiselectro-optic device can be operated by a method comprising the steps of(A) contacting a first location on a first surface of the device with astylus comprising a magnetic tip to cause a third type of particles tomigrate towards the first location, and (B) contacting again the firstlocation on the first surface of the device with the stylus comprising amagnetic tip to cause a second type of particles to migrate towards thefirst location. The method enables the user to write or draw in a deviceusing two different colors.

The dispersions of electrophoretic media according to the variousembodiments of the present invention may be encapsulated. Encapsulatedelectrophoretic media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically mobileparticles in a fluid medium, and a capsule wall surrounding the internalphase. Typically, the capsules are themselves held within a polymericbinder to form a coherent layer positioned between two electrodes. In amicrocell electrophoretic display, the charged particles and the fluidare not encapsulated within microcapsules but instead are retainedwithin a plurality of cavities formed within a carrier medium, typicallya polymeric film. Microcells may be formed either in a batchwise processor in a continuous roll-to-roll process as disclosed in U.S. Pat. No.6,933,098. The latter offers a continuous, low cost, high throughputmanufacturing technology for production of compartments. The microcellsmay be fabricated with embossing, photolithography, contact printing,vacuum forming, or other suitable methods. In this construction, themicrocells may be sandwiched between a light-transmissive electricallyconductive layer and an array of pixel electrodes. In one embodiment,the microcells are fabricated separately and then positioned between thelight-transmissive electrically conductive layer and an array of pixelelectrodes. For example, the microcell structure may be fabricated byembossing. The embossing is usually accomplished by a male mold, whichmay be in the form of a roller, plate or belt. The embossed compositionmay comprise a thermoplastic, thermoset or a precursor thereof. Theembossing process is typically carried out at a temperature higher thanthe glass transition temperature of the microcell material. A heatedmale mold or a heated housing substrate against which the mold pressesmay be used to control the embossing temperature and pressure. The malemold is usually formed of a metal such as nickel. Once formed, themicrocells are filled with the electrophoretic medium, The filledmicrocells are then sealed and the sealed microcell are laminatedbetween a light-transmissive electrically conductive layer and an arrayof pixel electrodes.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be used in the displays ofthe present invention.

The manufacture of a three-layer electro-optic display normally involvesat least one lamination operation. For example, in several of theaforementioned MIT and E Ink patents and applications, there isdescribed a process for manufacturing an encapsulated electrophoreticdisplay in which an encapsulated electrophoretic medium comprisingcapsules in a binder is coated on to a flexible substrate comprisingindium-tin-oxide (ITO) or a similar conductive coating (which acts asone electrode of the final display) on a plastic film, thecapsules/binder coating being dried to form a coherent layer of theelectrophoretic medium firmly adhered to the substrate. Separately, abackplane, containing the array of pixel electrodes and an appropriatearrangement of conductors to connect the pixel electrodes to drivecircuitry, is prepared. To form the final display, the substrate havingthe capsule/binder layer thereon is laminated to the backplane using alamination adhesive. (A very similar process can be used to prepare anelectrophoretic display usable with a stylus or similar movableelectrode by replacing the backplane with a simple protective layer,such as a plastic film, over which the stylus or other movable electrodecan slide.) In one preferred form of such a process, the backplane isitself flexible and is prepared by printing the pixel electrodes andconductors on a plastic film or other flexible substrate. A laminationtechnique for mass production of displays by this process is rolllamination using a lamination adhesive. Similar manufacturing techniquescan be used with other types of electro-optic displays. For example, amicrocell electrophoretic medium may be laminated to a backplane insubstantially the same manner as an encapsulated electrophoretic medium.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production.Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term “light-transmissive” should ofcourse be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a. thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly(ethylene terephthalate) (PET) films coated withaluminum or ITO are available commercially, for example as “aluminizedMylar” (“Mylar” is a Registered. Trade Mark) from du Pont de Nemours &Company, Wilmington Del., and such commercial materials may be used withgood results in the front plane laminate.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called “double release sheet”which is essentially a simplified version of the front plane laminate ofthe aforementioned U.S. Pat. No. 6,982,178. One form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

U.S. Pat. No. 7,839,564 describes a so-called “inverted front planelaminate”, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

In the processes described above, the lamination of the substratecarrying the electro-optic layer to the backplane may advantageously becarried out by vacuum lamination. Vacuum lamination is effective inexpelling air from between the two materials being laminated, thusavoiding unwanted air bubbles in the final display; such air bubbles mayintroduce undesirable artifacts in the images produced on the display.However, vacuum lamination of the two parts of an electro-optic displayin this manner imposes stringent requirements upon the laminationadhesive used, especially in the case of a display using an encapsulatedelectrophoretic medium. The lamination adhesive should have sufficientadhesive strength to bind the electro-optic layer to the layer(typically an electrode layer) to which it is to be laminated, and inthe case of an encapsulated electrophoretic medium, the adhesive shouldhave sufficient adhesive strength to mechanically hold the capsulestogether. If the electro-optic display is to be of a flexible type, theadhesive should have sufficient flexibility not to introduce defectsinto the display when the display is flexed. The lamination adhesiveshould have adequate flow properties at the lamination temperature toensure high quality lamination, and in this regard, the demands oflaminating encapsulated electrophoretic and some other types ofelectro-optic media are unusually difficult; the lamination has beconducted at a temperature of not more than about 130° C. since themedium cannot be exposed to substantially higher temperatures withoutdamage, but the flow of the adhesive must cope with the relativelyuneven surface of the capsule-containing layer, the surface of which isrendered irregular by the underlying capsules. The laminationtemperature should indeed be kept as low as possible, and roomtemperature lamination would be ideal, but no commercial adhesive hasbeen found which permits such room temperature lamination. Thelamination adhesive should be chemically compatible with all the othermaterials in the display.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications and improvements areintended to be within the spirit and scope of the technology describedin the application. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments described herein. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described. In addition, any combination of two ormore features, systems, articles, materials, kits, and/or methodsdescribed herein, if such features, systems, articles, materials, kits,and/or methods are not mutually inconsistent, is included within thescope of the present disclosure.

What is claimed is:
 1. An electro-optic device comprising: a firstsurface on a viewing side; a second surface on the opposite side of thefirst surface; an electrophoretic medium disposed between alight-transmissive electrically conductive layer and an array of pixelelectrodes and comprising a plurality of particles in a fluid, theplurality of particles comprising: a first type of particles having afirst color, a first charge of a first charge polarity; a second type ofparticles having a second color, a second charge of a second chargepolarity; and a third type of particles having a third color, a thirdcharge of the second charge polarity; wherein the first, second andthird colors are different from one another; the first charge polarityis opposite to the second charge polarity; the plurality of particlesare configured to migrate within the fluid in a direction responsive toan applied electric field; the third type of particles are configured tomigrate within the fluid in a direction responsive to an appliedmagnetic field gradient; the second type of particles have an electricfield threshold, such that: (a) application of a voltage potentialdifference between the light-transmissive electrically conductive layerand a pixel electrode to generate an electric field stronger than theelectric field threshold and having a polarity driving the second typeof particles adjacent to the light-transmissive electrically conductivelayer, will cause a pixel corresponding to the pixel electrode todisplay the second color at the first surface; (b) application of avoltage potential difference between the light-transmissive electricallyconductive layer and a pixel electrode to generate an electric fieldstronger than the electric field threshold and having a polarity drivingthe first type of particles adjacent to the light-transmissiveelectrically conductive layer, will cause a pixel corresponding to thepixel electrode to display the first color at the first surface; (c)once the first color is displayed at the first surface, application of avoltage potential difference between the light-transmissive electricallyconductive layer and a pixel electrode to generate an electric fieldweaker than the electric field threshold and having a polarity drivingthe third type of particles adjacent to the light-transmissiveelectrically conductive layer, will cause a pixel corresponding to thepixel electrode to display the third color.
 2. The electro-optic deviceof claim 1, wherein the second type of particles are not responsive to amagnetic field gradient. The electro-optic device of claim 1, whereinthe first color is white.
 4. The electro-optic device of claim 3,wherein the second color is black.
 5. The electro-optic device of claim1, wherein the average diameter of the third type of particles is largerthan the average diameter of the second type of particles.
 6. Theelectro-optic device of claim 1, wherein the fluid is a non-polarsolvent.
 7. The electro-optic device of claim 1, in combination with atouch sensor anti a digitizer.
 8. The electro-optic device of claim 7 incombination with a stylus.
 9. The electro-optic device and stylus ofclaim 8, wherein the stylus comprises a magnetic material proximate to afirst tip of the stylus.
 10. The electro-optic device and stylus ofclaim 9 further comprising a second stylus having a non-magnetic tip.11. The electro-optic device and stylus of claim 10, wherein the styluscomprises a second non-magnetic tip on an opposing end of the stylusrelative to the first tip.
 12. The electro-optic device and stylus ofclaim 11, wherein the device is configured to display at least one colorwhen the non-magnetic tip contacts a surface of the display.
 13. Theelectro-optic device and stylus of claim 12, wherein the device isconfigured to display at least two colors.
 14. A method of operating anelectro-optic device according to claim 1 comprising the steps of:contacting a first location on the first surface of the device with oneof a first and second stylus; applying an electric field to cause one ofthe first type and second type of particles to migrate towards the firstlocation; contacting a second location on the first surface of thedisplay with one of the first and second stylus; and applying a magneticfield gradient to cause the third type of particles to migrate towardsthe second location.
 15. A method of operating an electro-optic deviceaccording to claim 8 comprising the steps of: contacting a firstlocation on the first surface of the device with the stylus comprising amagnetic tip to cause the third type of particles to migrate towards thefirst location; contacting again the first location on the first surfaceof the device with the stylus comprising a magnetic tip to cause asecond type of particles to migrate towards the first location.
 16. Amethod of operating an electro-optic device, wherein the electro-opticdevice comprises (a) a first surface on a viewing side; (b) a secondsurface on the opposite side of the first surface; (c) artelectrophoretic medium disposed between a light-transmissiveelectrically conductive layer and an array of pixel electrodes andcomprising a plurality of particles in a fluid, the plurality ofparticles comprising (1) a first type of particles having a first color,a first charge of a first charge polarity; (2) a second type ofparticles having a second color, a second charge of a second chargepolarity; and (3) a third type of particles having a third color, athird charge of the second charge polarity; wherein the first, secondand third colors are different from one another; wherein the firstcharge polarity is opposite to the second charge polarity; wherein theplurality of particles are configured to migrate within the fluid in adirection responsive to an applied electric field; and wherein the thirdtype of particles are configured to migrate within the fluid in adirection responsive to an applied magnetic field gradient; comprisingthe steps of: contacting a first location on a first surface of thedevice with a stylus comprising a magnetic tip to cause a third type ofparticles to migrate towards the first location; contacting again thefirst location on the first surface of the device with the styluscomprising a magnetic tip to cause a second type of particles to migratetowards the first location.